As energy prices continue to rise, alternative energy sources become increasingly important. In particular, the use of waste methane (from renewable sources, such as municipal digesters and landfills) as a fuel is becoming increasingly widespread. Recent incentives offered by State and Federal Governments have led to the installation of more and more digester and landfill biogas power generation projects. As many developers of such projects have found, biogas is similar to crude oil in that it must be “refined” in order to use biogas as a reliable fuel. Biogas frequently contains high levels of moisture, high levels of hydrogen sulfide, and moderately high levels of halogenated contaminants. Most often, biogas also contains significant levels of organosilicons (siloxanes in particular), which are common additives to personal care products such as shaving cream, lipstick, hand cream, deodorants and hair styling products. Combustion of organosilicons forms silicon dioxide and other silicas or silicates. Silicon dioxide is the main ingredient in sand, and such silicates can damage power generation equipment. Because of the damage inflicted by silicates, it is desirable to remove organosilicons (a source of such silicates) from biogas, to prevent such damage and prolong the life and reliability of power generation equipment. Organosilicon levels above 50 parts per billion by volume (ppbv) in 5 biogas used as a fuel can cause severe damage to power generation equipment such as micro-turbines and turbines. Organosilicon levels above 100 ppbv in biogas used as a fuel can cause premature wear and damage to internal combustion engines used as generators.
While removal of organosilicons from biogas used as a fuel is clearly desirable, unfortunately such removal has proved to be quite challenging accomplish economically. Moreover, removal of all the organosilicons in a particular biogas is difficult, due to the wide variety of different organosilicons present in many biogas streams. Different varieties of organosilicons exhibit different molecular weights and different volatilities, complicating any removal strategy. Conventional removal methods employing activated carbon, silica gel and cold chilling lack either the ability to completely remove all of the various types of organosilicons present in a biogas stream, or are too costly. It would therefore be desirable to provide an economical technique for removing different varieties of organosilicons from biogas streams.
Non-regenerable systems, most often utilizing activated carbon or silica gel in “fixed” or stationary media beds, can only partially remove organosilicons from biogas, and even then such systems are operational only for relatively brief periods of time before requiring the media to be replaced. Activated carbon and silica gel are both negatively affected by moisture in the gas, significantly reducing their capability to remove organosilicons and almost completely eliminating their ability to remove halogenated chemical species. Moreover, highly contaminated biogases, such as those with volatile organic carbon (VOC) burdens above 400 parts per million by volume (ppmv) can cause a rapid heating of both activated carbon and silica gel media, thereby creating a dangerous condition that can lead to ignition of the organic materials picked up by the media. Non-regenerable biogas treatment systems also generate spent media, a waste product that requires replacement and disposal, generating additional expenses. Regenerable systems employing activated carbon or silica gel, with a classical stationary “deep bed” approach (i.e., including a bed of media several feet in depth), are unwieldy to operate due to heating and cooling cycle times associated with the regeneration of the media. They are also costly to operate, due to their relatively high energy consumption. In addition, such systems generally exhibit a relatively poor removal efficiency for organosilicons and halogenated organics. Regeneration of the adsorbent media also produces a waste stream, generally a foul smelling liquid organic/water waste stream that must be disposed of at an additional cost. Disposal 5 of such wastes further carries inherent risks of future liability if the ultimate disposal site requires cleanup. Moreover, both regenerable and non-regenerable systems employing activated carbon or silica gel in deep bed vessels require a significant amount of space, which may not be readily available. It would therefore be desirable to provide a regenerable system having a relatively small footprint, and which is capable of removing a large number of different organosilicons and VOCs, thereby minimizing any waste stream.
Another biogas treatment technique is cold chilling, which is based on the principle of lowering the temperature of the biogas to a temperature below the condensation point of the organosilicons and halogenated chemical species contained in the biogas. Such systems generally require a refrigeration unit capable of operating to as low as −20 degrees F, to effectively chill the biogas to −9 degrees F. Although these systems can remove many organosilicons and halogenated VOCs, they are ineffective on contaminants exhibiting very low boiling points and high vapor pressures. Because these systems operate below the freezing point of water, ice forms in the heat exchangers and the heat exchangers must periodically be thawed out. For this reason, duplicate systems must be installed to provide for continuous operation. Energy consumption, expressed as a “parasitic load,” is the highest with this type of biogas treatment equipment. Such systems produce a large volume of water waste and volatile chemical condensate wastes that must be disposed of at an additional cost. Furthermore, cold chilling systems also require a relatively significant amount of space for installation, which is not always readily available at potential development sites.
More recently, fluidized media bed systems have been introduced for control of VOC emissions and solvent recovery from air. Such systems generally utilize a relatively small sized particle of adsorbent material manufactured from pyrolized petroleum coke or synthetic resins. While effective for solvent recovery and to remove VOCs from air, such systems are not particularly effective at removing organosilicons and halogenated organics from biogas. In general, systems configured to remove contaminants from air include components than cannot readily withstand the harsh chemical conditions associated with the processing of biogas. As a result, rapid corrosion and failure of key components occurs. Furthermore, the moisture present in biogas can cause the relatively small adsorbent particles in such systems to conglomerate, degrading the fluidity of the media bed, which leads to system failure. In addition, an outside fuel source must be utilized to destroy the organics once they 5 are removed from the air stream, or energy must be used to condense the removed organics so they may be re-used or disposed of as a liquid waste stream.
Because such air purification technology is designed for relatively low pressure or ambient (i.e., atmospheric) pressure streams, the equipment cannot withstand the higher biogas pressures required by many types of power generation equipment. Even at relatively low pressures, distortion of rectangular process equipment components occurs, resulting in gas leaks. Biogas leaks pose several problems. Since biogas is a fuel and has a commercial value, gas leaks in treatment equipment can be expensive, as well as being dangerous. Biogas is also highly odiferous, containing condensable organics referred to as “skunk oil.” Thus, it is desirable to prevent gas leakage.
A significant drawback of existing fluidized media bed technology is a lack of adequate automation. Most projects involving the combustion of biogas for power generation require biogas systems to be operational with less than a 5% downtime. It would therefore be desirable to develop automated systems capable of operating with minimal downtime.
A drawback of the biogas treatment systems discussed above is that they generally are not able to attain the high purity level required by most biogas combustion equipment. Thus, it would be desirable to provide for a nominally complete removal of organosilicons and halogenated volatile chemicals.